Why hasn't someone built an XMR web-miner that is first & foremost made to replace captchas & reduce spam while also generating revenue?

I imagine the profit potential would be quite high for anyone who could implement their own solution & maybe like XMRig have a default donation value to get paid for creating thing.

It would be amazing if as a website owner I could have set difficulties to preform certain tasks & the server could send users work units for users to complete, obviously at a much lower difficulty but if they manage to actually complete the work or get a correct hash then it could be designed to automatically send that back. Increased revenue (maybe enough to replace ads), less spam, & no captchas for users, everyone would win & all this extra mining would increase the security of the Monero network.

Is there an obvious reason I'm not seeing as to why this hasn't been done?